1. Field of the Invention
The present invention relates to intrusion detection systems and particularly to safety systems used on industrial machines. This improvement relates to means for presetting an acceptable obstruction.
2. Description of the Prior Art
Intrusion detection systems employing light beams are well known and typified in U.S. Pat. Nos. 4,249,074 and 3,970,846. These systems employ light beam sources and photodetectors to create a screen, and then monitor the beams (photodetector output) to detect an intrusion. In U.S. Pat. No. 4,249,074 a system is proposed which detects an impermissible intrusion, such as an operator's hand, by counting the number of consecutive beams which are interrupted. However, a need exists for the system which allows a specific obstruction (i.e. work bench) or detects the absence of an obstruction (i.e. feed stock).